[V8 perf] Loop callback changing is throttling

Start by cloning the linked reproduction and running demo-1.ts and demo-2.ts under Node.js v24, comparing the callback-switching timings. Investigate the reported V8 polymorphic inline-cache behavior and verify the result against the BOX and LANCZOS cases. Done means changing callbacks no longer causes the repeated downsample2D calls to slow from the expected timings.

Version

v24.20.0

Platform

Windows 10

Subsystem

No response

What steps will reproduce the bug?

• Clone the repo https://github.com/AlttiRi/loop-callback-throttling-bug
• Install deps ("sharp")
• Run demo-1.ts and demo-2.ts

How often does it reproduce? Is there a required condition?

Always

What is the expected behavior? Why is that the expected behavior?

• BOX (~5 secs) ✅
• BOX (~5 secs) ✅
• LANCZOS (~8 secs) ✅
• LANCZOS (~8 secs) ✅

What do you see instead?

• BOX (~5 secs) ✅
• BOX (~5 secs) ✅
• LANCZOS (~30 secs) ❌
• LANCZOS (~30 secs) ❌

Additional information

I have downsample1D_V1 function (it's very unoptimized, but it works, and it's perfect for the demonstration), which executes the passed callback const weight = fn(x); billions of times (3_107_687_040 times per 1 downsample2D call).

There are 2 types of the passed callback function — lanczos3 and box:

function lanczos3(x: number): number {
  if (-3 <= x && x < 3) {
    return sinc(x) * sinc(x / 3);
  }
  return 0;
}

function sinc(x: number): number {
  if (x === 0) {
    return 1;
  }
  x = Math.PI * x;
  return Math.sin(x) / x;
}


function box(x: number): number {
  if (x > -0.5 && x <= 0.5) {
    return 1;
  }
  return 0;
}

I call downsample2D multiple times with the follow callback params (demo-1.ts):

• BOX (~5 secs) ✅
• BOX (~5 secs) ✅
• LANCZOS (~30 secs) ❌
• LANCZOS (~30 secs) ❌

The first 2 calls work with the expected speed, for ~5 second.
But when the callback is changed, downsample2D works multiple times slower than expected, for ~30 seconds.

In reverse order I also face the throttling (demo-2.ts):

• LANCZOS (~8 secs) ✅
• LANCZOS (~8 secs) ✅
• BOX (~30 secs) ❌
• BOX (~30 secs) ❌

The problem appears on both Node.js (24) and Deno (2.9).

Bun (1.3/1.4) works fine:

• BOX (~5 secs) ✅
• BOX (~5 secs) ✅
• LANCZOS (~8 secs) ✅
• LANCZOS (~8 secs) ✅

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Related keywords: performance v8 polymorphic inline cache